The pathology and diagnosis of diseases of the chest; illustrated especially by a rational exposition of their physical signs. With new researches on the sounds of the heart

Front matter

THE ah PATHOLOGY AND DIAQNOsIs ~*?.

an * a pats ‘: ea if ae OF | i ef Pe DISEASES OF THE CHES: A Rational Erposittion of their Phpsiral Stans,

WITH NEW RESEARCHES ON

PART I.—On THE PRINCIPLES OF THE PHYSICAL STUDY OF DISEASE OF THE CHEST . 2 é

Cuap. I.—On the Nature of Physical Signs. Definition—Application of the Senses to the Study of Disease, 1—2; Superior applicability of Hearing, 2—4; Nature and Properties of Sound ; Nature and Variety of Vibrations; Differences and Sources of Sounds ; Conduction of Sound; Reflection of Sound, 4—10.

Cuap. I1.—On the Physical Examination of the Chest Utility of this Method for the Chest, 11, 12.

Sect. 1.—Ocular and Manual Examination, 12—16.

Sect. 11.—On Percussion. Causes of Resonance, 16, 17; Causes of Modification, 18; Methods of Percussion, 18 ; Mediate Percussion, 19—20; Practice, 21.

SEcT. 111.—On Auscultation, 22; Sounds of Respiration, vesicular, puerile, bronchial, cavernous, 23—28; Rhonchi, dry, mucous, cavernous, crepitant, 28—33; Auscultation of the Cough, 33; Sounds of the Voice, Laryngophony, Bronchophony, Pectoriloquy, 33—38.

Sect. 1v.—Methods of Auscultation, Immediate Auscultation, 88; Mediate, 39, 40; Use of the Stethoscope, 42, 43.

Sect. v.—On Expectoration, 44—46.

PART JI.—On tHE Puysicat Sicns oF DISEASES OF THE LuNGS AND PLEURA.

Cuap. I.—Diseases of the Air Tubes : Sect. 1.—Acute Bronchitis, Pathology, Signs, Rhonchi, &c. 48—53; Chronic Bronchitis, Signs, Distinction from Phthisis, Dilatation of the Bronchi, 53—57.

PAGE.

1—10

11—46

47 —65 v1 CONTENTS.

PAGE.

Secr. 11.—-Pituitous Catarrh, 57—59.

Sect. 111.—Dry Catarrh, 59, 60.

Sect. tv.—Pertussis, Croup, Ulcers of the Bronchi, &c. 60—62.

Sect. v.—Spasmodic Asthma; Pathology, Signs, Causes, 63—65.

Cuapr. II.— Diseases affecting the tissue of the Lungs : . 66—87 Sect. 1.— Peripneumony, Ist stage; Pathology, Signs, Crepitant Rhonchus, &c. 66—69; 2nd stage—Pathology, Anatomy, Signs, Bronchial Respiration, &c. 69—74; 3rd stage— Abscess, Gangrene, 74; Progress and Removal of Peripneumony, 75—77; Varieties and Complications, 77—79. Sect. 11.—Emphysema of the Lungs; Causes, Pathology, and Signs, 79—83; Interlobular Emphysema, 83.

Sect. 111.—(idema of the Lungs, 84, 85.

SEctT. 1v.—Pulmonary Apoplexy, Hemoptysis, 85—87.

CHAP. III.—Diseases of the Pleura . 88—122

Sect. 1.—Acute Pleurisy ; Symptoms, Puthdlogy: ma Signs, Gigophony, 88—93; Extensive Effusion, Signs, latent Effects, 94—96 ; Signs of Termination, 97; Effects of Inflammation on Tissues, Acute Inflammation, Adhesions, &c. 98—102; Hemorrhagic Pleurisy, Contraction of the Chest, 102—104; Chronic Pleurisy, Empyema, 104—106.

Sect. 11.—Pleuropneumonia; Varieties and Signs ; Buzzing (gophony, 107—110.

Sect. 111.—Hydrothorax, 110.

Srect. 1v.— Hemothorax, 111.

SEcT. v.—Pneumothorax; Varieties; Signs; Cause of Metallic Tinkling and its Varieties, 112—122.

Cuap. IV.—Phthisis Pulmonalis , A 1938-158 Sect. 1.—Pathology. Gyranulations, 123; saagahen 124; Crude and Mature Tubercle, 125; Tuberculous Excavations, 125—127; Nature of these Changes, and their Relation to Inflammation, 127—129; Illustrations of the Origin of Tubercle,

Softening, &c. 129—1385; Inefficacy of Medicine, 135, 136.

Sect. 11.—Signs of Phthisis. Induration, 137—140; Signs of Softening and Excavation, 141—147; Distinction from Dilated Bronchi, 148; Perforation of the Pleura, 149, 150; Expectoration in Phthisis, 150, 15] ; Cicatrization of Cavities, and Cure, 152, 153.

CONTENTS. Vii

PAGE.

Sect. II].—Encephaloid Disease, Fungus Hematodes, Melanosis, Spurious Melanosis, 153—156.

Sect. I[V.—Diseases of Bronchial Glands, 157, 158. PART II1.—AvuscuttTation oF THE HEART.

Introductory Remarks bs : : . - 159—160 Cuap. I.—Auscultation of the Heart in Health ; . 160—162

Sounds, 160; Impulse, 161; Healthy Varieties, 162.

Cuap. IIl.—An Inquiry respecting the Motions and Sounds of the Heart : ‘ ; : ; : . 163—179 Consideration of former opinions and researches on the subject, 163—168; Tenable Conclusions, 166—169; Doubtful points stated, 169; Experiments to decide these, 170—174; Conclusions from these Experiments, 175, 176 ; Remarks on the mode of production of the sounds, 176—178; Remarks on M. Majendie’s views, 178, 179.

Cuap. I11.—Auscultation of the Heart in disease : . 180—206

Difficulties of the subject, 180; Dilatation of the Heart, Causes and Signs, 181, 182; Hypertrophy, Causes and Signs, 182—185; Hypertrophy and Dilatation, 185—187; Pericarditis, Pathology, Signs, Effusion and Adhesions, 187—190; Softening of the Heart, 190—191; Diseases of the Valves of the Heart, general Effects and Symptoms, 191, 192; Causes of the Bellows, Filing, and Sawing, Murmurs, 193—195; Purring Tremor, 195; Diseases of the Aortic Valves, Obstructive, 197; Regurgitant, 198; Mitral Valve, Obstructive, 198; Regurgitant, 199; Pulmonary and Tricuspid Valves, 200, 201; Nervous Palpitation, 201, 202; Aneurisms of the Aorta, 2038 —205; Dilatation, 205.

EXPLANATION OF THE PLATES.

Construction of the Stethoscope : ; : . 207—209 Table I.—Thoracic Regions, with Plate IT.

Table I].—View of the Physical Signs.

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Preface

Tue gratifying manner in which the former editions _ of this work have been received, has encouraged me to devote much attention and care to render it, in its present form, more worthy of the same kind reception. The progress which patient and comprehensive study constantly ensures to pathological science, has furnished many important additions from the works of various distinguished labourers in this field; and these, together with the later results of my own experience and study, have greatly improved, and nearly doubled, the matter in the present volume. Of these additions, | may mention, that the sections On the Ocular and Manual Examination of the Chest ; On Expectoration; On Encephaloid Disease, Melanosis, §c.; On Diseases of the Bronchial Glands ; and the whole of Part III. On the Auscultation of the Heart,—are new; and large additions have been made to the sections on Bronchitis, Peripneumony, Pleurisy, Pneumothorax, and Phthisis.

The experimental researches on the sounds of the heart, contained in Part III. will, it is hoped, give a greater accuracy and precision to the pathology of xX PREFACE.

that organ, than it has hitherto acquired; and although in the subsequent chapter on this subject I have expressed much doubt about the diagnosis of certain lesions, yet the attempt to distinguish and separate the points that are clear and useful, may lighten the labour and disappointment of the student, and assist future inquirers to clear up the remaining difficulties.

It has gratified me to find that the connection which I have endeavoured to establish between physical diagnosis and pathology, by tracing signs to their mechanism, and the mechanism through its variations to the production of the signs, has been generally approved ofand adopted, and I have now only to repeat my conviction, strengthened by further experience, that the easiest way to study auscultation, and the only sure way to profit by it, is through a knowledge of pathology, and of the physical laws of sound.

Within the last few years, auscultation has been rapidly advanced in the estimation of the profession ; and I think this is much to be ascribed to the more rational and moderate manner in which its pretensions have been urged. Instead of being blindly put forth as infallibly indicating such and such diseases, its signs are now appreciated only as they inform us of certain physical changes in the organs within the chest; whilst the aid of our other senses, a due attention to the general symptoms, and a careful reflection on all these evidences, are also required,

PREFACE. Xl to enlighten us in our study of disease, and to suggest remedies for its removal. It is with a view to prevent the student from relying too exclusively on the physical signs, that I have in the present edition added a short summary of the general symptoms of each disease, which in very many instances are more important in guiding our practice, than the local signs; and for the same reason, and also because the principles of pathology and diagnosis are more fully considered, I have somewhat altered the title of the work, hoping that it may still prove an easy guide to a knowledge of auscultation, and at the same time explain the general pathology and diagnosis of Diseases of the Chest.

PREFACE

TO THE FIRST EDITION.

A DISCOVERY, a new doctrine, or an innovation of any kind, produces a curious agitation in the public mind, which, in a remarkable manner, illustrates the paradoxical and heterogeneous composition of human character. Tossed to and fro by the exertions of its opponents, and of its scarcely less opposing ultra-partisans,it resembles a pendulum; and vibrating irregularly between many disturbing forces, it is driven out of the real sphere of its importance, and from that true point of utility to which its intrinsic weight and worth would cause it to gravitate.

More particularly has this happened in medicine, which, having few standard or fixed points to steady it, has been ever too much at the mercy of contending opinions. The localization of diseases is a characteristic doctrine of the present day; and assuredly such a system would be the most scientific, that could trace the multifarious forms of disease to a few simple primary lesions of tissue, or well defined alterations of function ; and that plan of practice the most efficacious that could concentrate its efforts against the very root of evil, and stop atits very spring-source the current of disorder. But we are far from having attained such a perfection; and let caution, therefore, remind us, that hastily to follow a light, which, however pure and real, is yet at distance too remote

PREFACE TO THE FIRST EDITION. Xill to shed its rays upon our paths, is scarcely less dangerous, than to chase an empty ignis fatuus; that to grasp at an object, however perfect and substantial, so far beyond our present reach, is not less futile than to catch at an illusory shadow.

The local study of diseases must not, therefore, remove our attention from their general phenomena ; our examination of their physical nature must not exclude the consideration of many constitutional effects, that by reaction may often become converted into causes; and still less should physical signs of doubtful import make us neglect obvious disorder of the system.

But, thus limited, the local study of diseases is more advantageous than the knowledge of their general forms; an examination of their physical signs, when possible, more useful than the perplexing consideration of a host of uncertain and fallacious constitutional symptoms ; and when physical signs are wanting, or beyond the sphere of our observation, those constitutional ones are our best guides, which most nearly depend on the physical and unchangeable character of the disease. For the local study of a disease acquaints us with its proximate and essential cause, and this knowledge su S means for its removal ; and by a study of its physical signs, and of those general ones most allied to them, we obtain the most certain method of discovering its existence, and of distinguishing its character.

Further than this, I shall not expatiate on the advantages or disadvantages of the new methods of diagnosis of diseases of the chest. They are now too well understood and appreciated to be in danger: of

XIV PREFACE TO THE FIRST EDITION.

yielding to the opposition of prejudice, or of falling into oblivion through neglect. Too many ears have been opened to the language of disease, to suffer its warnings to be lost without a listener; too many minds are convinced of the truth of its admonitions, to permit them to pass, as hitherto, unheeded. Those who are disposed to study the signs of auscultation and percussion, will soon find in that study the proofs of their merit and importance; those who will not examine them, are not likely to be more moved by any commendations that I could bestow, than by those that have already been written in their favour.

The “ Traité de ’Auscultation Médiate,” and the perfect translation of Dr. Forbes, are at length generally appreciated, even in this country, slow to award its meed of praise. The homage paid to the talents of the author, gives me a gratification that almost seems personal; and I doubt not that this feeling is shared by others of his pupils, in whom his urbane and amiable deportment created a sincere regard for the man, as his great mental abilities excited our respect. His great talents are known to the public through the medium of his writings; but those who attended his clanzque can only appreciate the wonderful acuteness of his perception, and faculty for observation, that enabled him to carry his discovery to the degree of perfection in which he left it; and they, above all, witnessed, felt, and profited by the solicitous interest which he showed to make others partake of its inestimable advantages. They felt in his death the loss of a friend ; as science had to deplore the loss of his talents: he has wrought a good work for both; the feeling shall last while they last ;

PREFACE TO THE FIRST EDITION. XV science has recorded his name on her tablets for ever :—

Let me say a few words on the objects and plan of the present work.

I have ever found in practice, and it is perfectly conformable to reason, that the easiest and most agreeable way to study physical signs, and to attain the surest criterion of their value and importance, is by considering how they are caused, or what are the relations in which they stand to the physiological and pathological states that produce them. Attempts to discover the rationale of the general symptoms of disease, have been as unsuccessful as our knowledge of the functions or properties on which they depend is scanty and imperfect ; and inquiries of this kind have been proportionately unsatisfactory and unprofitable. But physical signs stand on the broad and intelligible basis of physical laws, and are as readily explained as other simple phenomena illustrated by natural philosophy. It has been my endeavour to exhibit them, as far as possible, in this intelligible view; to show the mechanism by which the signs are produced, and the manner in which, according to fixed laws, they result as phenomena ; to make a knowledge of the pathology predicate the signs, and a knowledge of the signs indicate the pathology ; and by thus familiarizing the mind with their principles, to enable it to understand the multifarious forms which, by combination, these signs may assume, and to judge of the corresponding physical changes that modify or produce them.

Xvl PREFACE TO THE FIRST EDITION.

I have not refrained, when the subject seemed to require it, or where I had any new view to offer, from entering on some questions of general pathology. I am not clear that I have been judicious in so doing; for the slight views, that I have given of these questions, may be deemed too superficial and unsupported to be satisfactory ; and had they been developed more fully, it would have completely changed the size and nature of the work. ‘These opinions, as well as my acquaintance with the physical signs, are the result of some extent of study and observation, prosecuted chiefly in the wards of La Charité, where Laennec taught, and Andral prosecuted his labours. Most of the facts which I have described have appeared in the works of these illustrious men; and wherever my experience has not enabled me to give the same as the result of my own observation, I have referred to their competent authority. Where, in point of fact, or opinion, I have differed from them or from others, I would wish my dissent to be viewed rather as a question to be answered by others, than as in itself superseding former observations or opinions.

London, May 21, 1828.

The reader is particularly requested to correct with the pen the following Errata, as some of them materially injure the sense.

Page 96, line 23, for habitual read mutual.

— 118 — 19, for or read on.

— 122 — 5, for is read are.

— 185 — 21, for alternatives read alteratives.

— 167 —- 9 from bottom, (note) for distillation read dilatation. — 168 —_ 3, (note) for intimate read imitate.

— 185 — 16, erase the comma after blood.

— 198 —_ 3, for shifting read whiffing.

— tb. last line, for de read is.

.

THE

PATHOLOGY AND DIAGNOSIS,

PART IL—CHAP. I.

ON THE PHYSICAL SIGNS OF DISEASE.

By physical signs I mean such as depend on the direct operation of known laws of natural philosophy on our organs of sensation. As they are produced by the physical state or condition of a part, they become indications of that state or condition, as certain, as the laws, of which they are exemplifications, are unerring and sure: and the physical state of a part of the body may be ascertained with more or less certainty, as its physical signs, or relations to these natural laws, are more or less appreciable by our senses.

The organs of vision, impressed by forms and properties in relation to light, and perfected by the immediate correction of touch, are, both by nature and habit, calculated to give us a more perfect knowledge of external objects, that can be derived from the other senses. But the number of diseases that come under the cognizance of vision is very limited, as by far the greater part of the body is excluded from its sphere. Derangements of the surface, and of the openings of some of the passages to the interior, can alone be subjected to the direct examination of the eye. Mediately, physical changes of internal organs can be perceived by sight only, when their size,

B g ON THE PHYSICAL SIGNS OF DISEASE.

form, or position, is so far altered as to cause displacement of some external part; and the knowledge that such a sign gives us, although scanty, is often valuable.

The sense of touch, or tact, will, in the same cases, furnish us with further knowledge as to the form, substance, and constitution of a diseased part; and when perfected by experience, may frequently discover organic changes that are altogether imperceptible to sight.

The sense of smell is more rarely qualified to distinguish disease; as its impressions can only be conveyed through the medium of air; yet we shall find that cases are not wanting in which this sense may assist us in diagnosis.

Sound, as it may be both generated and propagated in every form of matter, solid, liquid, and aeriform, may be therefore considered a mean of examination of parts removed from sight and tact, more promising as its sphere is less limited. It is requisite, however, that the object of examination be capable of producing or transmitting awdible sound; and that changes in the part, cause in the sound thus produced or transmitted, corresponding changes that may be appreciated by the ear. The relations of the organ of hearing to the qualities of external objects, are, in ordinary life, much less exercised than those of tact and vision. Yet continual experience proves to us that the substance or consistence of simple objects is, in some measure, declared by the sound which they emit when struck. The sound of liquids in contact with air, is familiarly distinguished from that of solids in the same medium, and a little more attention discovers the varied sounds which air in motion produces in contact with solids in different forms. The rushing of the torrent, the murmur of the brook, the whistling of the wind, the howl of the tempest, the rustling of leaves, the rumbling of carriages, the trampling of horses, are instances out of thousands in which we recognise in the sound the nature of its physical cause; and the mental process is here

ON THE PHYSICAL SIGNS OF DISEASE. 3 so simple, that the association of the sound with the object that produces it, is scarcely more than an instinctive perception. Such scanty knowledge of the relations of sound suffices for the common purposes of life: to study them more closely, with a view to discover the nature of objects, were a work of supererogation whilst sight and tact are capable of giving us much more perfect and certain information. But an individual deprived of sight substitutes a perfection of tact, and of hearing and distinguishing sounds, which, in a great degree, compensates for his want of vision. So likewise may we, with equal advantage, so perfect our sense of hearing, as to make its indications available to instruct us of objects beyond the sphere of tact andvision. Now such perfection must in great measure depend on the practice of each individual, as a knowledge of simple sensations cannot be transferred by description; but the study may be much assisted and simplified by a general knowledge of the chief laws according to which sound is produced and propagated. Since the publication of the first edition of this work, in 1828, the science of acoustics has been more fully studied, both on its own account, and for the illustrations which it affords to optics, now also a science of vibrations. The student may therefore with greater confidence be referred to the authorities named below ;* but there are still some properties of sound not developed in other works, and, as they are essential to a right understanding of

* Sir J. Herschell’s article on Sound, in the Encyclopzedia Metropolitana, 1830. The article Acoustics, by Sir J. Leslie, in tbe Edinburgh Encyclopedia, 18380. Dr. Arnott’s Elements of Physics, Vol I. 1833: this valuable work will save students from the gross error, hitherto lamentably prevalent, of attempting to understand the complexities of the animal machine, without a previous knowledge of the laws of simpler matter. Some interesting papers on sound, by M. Savart, will be found in the “ Annales de Chimie,” Vols. 14, 24, 30, &c. and I may refer those interested in the matter to my own communication to the physical section of the British Association for the Advancement of Science, at their late meeting at Edinburgh, and published in the Philosophical Magazine, January, 1835.

pe

A ON THE PROPERTIES OF SOUND.

the acoustic phenomena of the chest, I shall retain, with some alterations, my original remarks on this subject.

ON THE NATURE AND PROPERTIES OF SOUND.

Sound, physiologically speaking, is an impression communicated to our sense of hearing by certain impulses of matter. Sound, in a physical sense, may be defined as motion of a certain velocity resisted with a certain force. The moving and the resisting forces, acting in opposite ways, constitute the vibrations of sound. All matter is susceptible of sonorous vibrations ; but the degrees of this susceptibility are as varied as matter is diversified in form and nature. As ageneral rule it may be stated, that it is in proportion to the strength and uniformity of the molecular elasticity in the matter.

By the term molecular elasticity, is meant that force by which the molecules of a body are held at a certain distance from each other, and resist any effort to displace them from it. Thus, glass and steel may be said to possess molecular elasticity in a powerful degree, because any external impulse is instantaneously communicated from particle to particle throughout their whole mass; and it is not lost or broken by the yielding or displacement of the molecules at the point struck. Air, and other fluids, on the other hand, cannot readily be thrown into vibrations, unless the impulse be very abrupt, or applied to some extent of surface, by which it becomes communicated to many particles at once.

I conceive that the motion of matter, producing sound, should be considered as molecular, although the result is the vibration of the mass. Let it be represented thus :—An impulse being impinged on certain molecules, momentarily overcomes the resistance of their inertia, and causes them to start ‘from their place ; that force of repulsion, which, existing between the molecules, resists the attempt to approximate them, transfers the impulse from molecule to molecule, and thus ex-

ON THE PROPERTIES OF SOUND. o tends it throughout the mass. The impulse that forced these molecules from their position, being overcome by the reaction of the elastic forces (attractive and repulsive), these forces drive them back.to beyond their proper station, whence, from the same cause, they again spring back; until, by a series of these alternating vibratory motions, the disturbing force is lost.

The assimilatory or propagating power, then, of these vibrations, depends on the molecular elasticity of the body; that is to say, on the repulsive and attractive forces that subsist between the molecules of which it is composed ; and in proportion as these forces are strong to re-act on a mechanical impulse, they will convert that impulse into a sonorous vibration. It is likewise apparent that uniformity, or equality of molecular elasticity, favours the propagation of sonorous vibrations. For if the elasticity of some molecules be less than that of others, the reaction being less, will produce vibrations not consentaneous with those of the others, and may impair, or even destroy them. Let us illustrate this by the vibrations of pendula. Suppose a number of pendula suspended in a line, and in the act of vibration: if these pendula are of the same length, the vibrations will be equal and consentaneous, and will neither interfere with, nor interrupt each other. Such are the vibrations in bodies whose molecular elasticity is uniform. But suppose the pendula of different lengths, and the vibrations, therefore, unequal, the motions would then interfere with and neutralize each other, and this the more effectually, the more varied and irregular they are.*

* Lest it should be supposed that, in these and subsequent illustrations, I have borrowed, without sufficient acknowledgement, from Sir J. Herschell’s Essay, before alluded to, I beg to state, that the above explanations were published nearly in their present form in the first edition of this work, about three years before Sir J. Herschell’s paper appeared. I have not the presumption to suppose that that distinguished philosopher ever-saw them; but it is satisfactory and gratifying to me, that he has advanced views and modes of illustration very similar to mine.

6 ON THE PROPERTIES OF SOUND.

There are, however, some vibrations that, although they are not synchronous, nevertheless promote each other, and these constitute what are called harmonic sounds. To show how this is effected, let us again refer to the pendulum. We have already remarked that pendula of the same length vibrate synchronously, and may, therefore, promote and strengthen each other. This isthe harmony of unison. Suppose one pendulum half the length of the other; it makes double the number of vibrations in the same space of time, and, being regularly in the same ratio of striking two for every one of the other, the vibrations do not counteract each other. This concord or harmony of vibrations of sound, produces the harmonic note of the octave. The same illustration will enable us to conceive the harmonics of the fifth, the fourth, and the third; the ratio of their vibrations being as 3 to 2, 4 to 3, and 5 to 4, of the key note ; and in like manner of other harmonics. These illustrations will render intelligible not only the simple sounds of separate bodies, but also the various compound noises which we are accustomed to hear; for, owing to the unequal molecular elasticity of the bodies in which they are produced, these sounds are always compound, and consist of a variety of vibrations, which may increase or neutralize each other according as the arithmetical relations of their motions harmonize or disagree: in the former case the sound will bea prolonged tone; in the latter a mere stroke or knock.

The propagation or conduction of sound from body to body is subject to the same rule; and, in fact, it consists in the transmission of the same impulse, producing sonorous vibrations from one body to another. A sound will, therefore, ceteris paribus, be best conducted by those bodies which approach in degree and strength of molecular elasticity the body in which that sound is generated. Thus a sound produced in air will be best propagated by air; one produced in a solid will be most completely conducted by a solid of the same den-

ON THE PROPERTIES OF SOUND. 4 sity and hardness, &c. On the other hand, bodies, very different in density, receive and transmit sonorous vibrations very imperfectly. hus air transmits, in a very impaired degree, the sounds produced in dense bodies, such as metals; and the sonorous vibrations of air are scarcely received by dense bodies.

The sounds produced by the collision of solids, and transmitted to us through air, are, nevertheless, among the Joudest that we hear; but this is by reason of the law before stated, that those bodies are most susceptible of sonorous vibrations, in which the molecular elasticity is greatest, as well as most uniform; and such sounds are incomparably louder when heard through solids, instead of through air. The transfer of sonorous vibrations may, however, be greatly favoured in another way, by bringing a large surface of the solid vibrating body in contact with the air, and otherwise modifying its form, as in the case of bells, &c. This is a separate branch of acoustics, and is not sufficiently connected with our subject to require notice here.

There are many substances that prove bad generators or conductors of sound, from their being of unequal density ; and those are worst in which this inequality is greatest. Linen and woollen stuffs are examples: the threads of which they are composed, leave interstices, which contain air very different in density from the solid fibres. In paper and pasteboard, the same fibres pressed closer together, and forming a more _ solid mass, become a far better conductor. The same is the case with all spongy bodies.